1. Field of the Invention
The present invention relates generally to a camera and more particularly to a focal plane shutter unit of the camera.
2. Related Background Art
FIG. 1 illustrates configurations of an earlier technological camera body and an earlier technological focal plane shutter unit. A shutter baseplate 1 has an aperture 1a. The numeral 2 designates a shutter mechanism portion. A cover plate 3 is formed with an aperture 3a. A leading curtain element 4 and a trailing curtain element 5 are disposed in between the shutter baseplate 1 and the cover plate 3. A shutter unit 6 comprises the shutter baseplate 1, the shutter mechanism portion 2, the cover plate 3, the leading curtain element 4 and the trailing curtain element 5. A camera body 7 has an aperture 7a for defining a photographic image plane. A film is, when photographed, regulated in terms of its position by two lengths of outside-of-film rails 7b provided in the camera body 7 with respect to the aperture 7a in up-and-down directions of the camera, which directions are orthogonal to a film advancing direction. Further, the film is set in a tunnel 8 formed of a difference in level between an inside-of-film rail 7c and the outside-of-film rail 7b, thereby regulating the position thereof in an optical-axis direction. Designated also by 7d is a film cartridge chamber, and 7e represents a spool chamber for winding the film.
FIG. 2 is also a vertical sectional view illustrating an earlier technological lens interchangeable type single-lens reflex camera. A mirror box 9 for housing a mirror for a finder optical system is fixed to the body 7 with screws 10a, 10b. A bayonet 11 having an interchangeable lens mounting fiducial surface 11a is fixed to the mirror box 9 with a screw 10c. A blade chamber 12 defined by the shutter baseplate 1 and the shutter cover plate 3 is disposed in between the mirror box 9 and the camera body 7, and is fixed to the mirror box 9 or the camera body 7.
In the above construction, the shutter is disposed in front of the aperture 7a formed in the camera body 7. Considering a shutter efficiency, however, it is preferable that a distance between a film surface and shutter blades be short. Hence, there is no alternative but to decrease a thickness of the cover plate 3. Accordingly, it is impossible to expect a high rigidity of the cover plate 3, and, if large in terms of a shutter charge power of a high-speed shutter, or if short of rigidity because of the shutter baseplate 1 being formed by plastic molding, a blade traveling space is narrowed due to the deformations of the cover plate 3 and of the shutter baseplate 1 as well. This results in such a problem as to exert an adverse influence on the shutter speed.
Further, according to the above construction, a focal plane in the camera is secured by controlling distances (rail-back dimensions) from the bayonet fiducial surface 11a to the inside-of-film rail 7c and the outside-of-film rail 7b. These distances are given by a sum of a thickness of the bayonet 11, a dimension from a bayonet fitting surface 9a to a body fitting surface 9b of the mirror box 9, and dimensions from a mirror box fitting surface 7f to the inside-of-film rail 7c and the outside-of-film rail 7b. If these parts are molded of plastic, as far as a single piece of part is concerned, the dimensions can be controlled with a high accuracy. The deformation is caused by attaching other parts such as an external cover, and it is therefore difficult to obtain the satisfactory rail-back dimensions. Therefore, the step hitherto taken is that the rail-back dimensions are controlled by cutting out the inside- and outside-of-film rails 7c, 7b by machining on the basis of the bayonet surface 11a after completing the assembly. This step might be, however, a factor of increasing the costs. Further, the camera body is fitted with a grip, a tripod mount, a suspension ring, etc. and thereby receives a large external force. This might consequently lead to a problem, wherein an error in the rail-back is produced because of the body 7 being deformed by applying the external force thereto in spite of the fact that the rail machining has been done.